The invention relates to a piston arrangement for a dual-stage piston compressor, having a crankshaft, several cylinders with pistons operating therein, two or more low-pressure stages and at least one high-pressure stage being formed as well as a piston compressor for rail vehicles with such a piston arrangement.
The arrangement according to German Patent Document DE-PS 765 994 is characterized in that the cylinders and the crank throws are designed such that the forces of due to inertia are balanced as well as possible. Gas forces are not mentioned as vibration-exciting components. An assignment of the individual cylinders to a respective compressor stage does not take place in this citation. A piston compressor having a crankshaft, several cylinders and pistons operating therein is known, for example, from German Patent Document DE-PS 765 994.
Light-weight designs are increasingly used in the construction of rail vehicles. Modern light-weight car body structures made, for example, of extruded aluminum profiles or support structures made of thin metal sheet frequently have natural frequencies close to the rotational speed of the compressor of the air supply system. The use of piston compressors is often not possible in the case of such constructions because the permissible structure-borne noise level is frequently exceeded.
This is a result of the fact that, based on their construction, piston engines generate inertia forces and moments caused by the oscillating masses at the crank mechanism as well as moments resulting from the gas forces. Particularly in the case of the dual-stage piston compressors frequently used in the rail vehicle field, a very non-uniform torque will occur. As indicated by the analysis of a typical load moment of such a compressor, the predominant fraction of the load moment corresponds to the rotary frequency of the piston engine which is frequently in the range of from 20 to 30 Hz. These frequencies, in turn, are very easily noticeable to a person situated in the vehicle occupant compartment because, for example, the natural frequency of legs with stretched-out knees may amount to approximately 20 Hz.
In coordination with the engine, the above-described load moment of a piston compressor generates an exciting torque about the axis of rotation of the compressor. The moment of inertia of a conventional piston compressor unit is significantly lower about the axis of rotation than about other axes. Because the transmission mode of an elastic bearing about the longitudinal axis of the compressor, as a rule, is closer to the rotary frequency than, for example, the vertical mode, which plays a greater role for the transmission of inertia forces, this torsional vibration is, as a rule, not insulated as well as other exciter components.
It is an object of the invention to provide a piston compressor engine which avoids the above-described disadvantages.
According to the invention, this problem is solved by a drastic reduction of the fraction of the first order in the load moment resulting predominantly from the gas forces in that, as a result of an unusual piston arrangement, two or more low-pressure stages are superimposed in an in-phase manner and operate offset by approximately 180 (degrees? translator) with respect to the high-pressure stage. Constructively, this is achieved in that, in the case of a piston arrangement for a dual-stage piston compressor having a crankshaft and several cylinders with pistons operating therein, in which case two or more low-pressure cylinders and at least one high-pressure cylinder are constructed, the two or three or more low-pressure cylinders are arranged with respect to the high-pressure cylinders such that the two or more low-pressure cylinders compress in-phase or offset by less than xc2x115 and offset by 180xc2x120 with respect to one or more high-pressure cylinders.
The inventors have recognized that also, as a result of the phase shift of all low-pressure cylinders with respect to one or more high-pressure cylinders, a drastic reduction of the first order resulting from the torque diagram is achieved and thus a drastic reduction of the vibration-exciting torque about the axis of rotation of the compressor.
In a first embodiment of the invention, the piston arrangement is an oil-lubricated piston arrangement.
However, it is particularly preferable that the piston arrangement is an xe2x80x9coil-freexe2x80x9d dry-running piston arrangement. In a special further development of the invention, the piston arrangement is constructed as a 3-cylinder arrangement with two low-pressure cylinders and one high-pressure cylinder, an additional low-pressure cylinder being situated opposite a high-pressure cylinder. Such an arrangement is particularly installation-space saving. Naturally, 4, 5 or 6-cylinder arrangements using the teaching according to the invention are also conceivable.
In an advantageous embodiment, by means of using heavy pistons, the pressure peaks in the torque diagram can clearly be reduced because an increased kinetic energy of the piston is converted to compression work. In particular, the pistons of the cylinders should have such a large mass that the pressure peaks in the tangential force diagram are reduced, in which case the inertia forces entering the tangential force diagram with respect to the pressure peak are in the rotational speed range of 1,000 l/min to 2,000 l/min typical of piston compressors, particularly 1,500 l/min higher than 15% of the gas forces with respect to the pressure peak. In the present application, the tangential force diagram is the torque course/crank throw.
In an advantageous embodiment, it is provided that, for example, in a 3-cylinder arrangement, the masses of the low-pressure cylinder situated on the side of the high-pressure cylinder, specifically the piston mass and/or connecting rod mass are selected such that they balance the opposite low-pressure piston as well as the high-pressure piston which are both disposed on the same crankshaft throw. In this case, the balancing can take place at the piston as well as at the connecting rod. As a result of the increase of the piston mass resulting from the balancing of masses, the bearing load at the connecting rod is reduced.
In addition to the balancing of masses by means of additional masses, it is also possible to balance the oscillating mass by means of a dummy piston running along. In the present application, a dummy piston is a piston which carries out no compression work.
Advantageously, the pistons are arranged such that the low-pressure pistons take in by way of the crankcase in an in-phase manner, during the intake operation, the two low-pressure stages plunging into the crankcase pushing the air into the compression space. As a result, the intake vacuum in the low-pressure stage is reduced and the charging is improved. In a particularly advantageous embodiment, this effect is intensified by the use of a return valve at the inlet connection piece from the air filter housing to the crankcase. The arrangement of a return valve improves the efficiency particularly of a dry-running piston arrangement.
In addition to the piston arrangement, the invention also provides a piston compressor, particularly for rail vehicles comprising such a piston arrangement, which piston compressor advantageously comprises an electric-motor drive. The piston arrangement can also be used in the case of compressed-air generating systems in the industrial field.